The present invention relates to an image forming apparatus, such as a copier, a facsimile, a multi-functioned apparatus having functions thereof, etc., which employs an electro-photographic method.
In the image forming apparatus employing the electro-photographic method, a toner image is acquired by developing the electrostatic latent image formed on the photoreceptor element.
When the abovementioned latent image is formed as a solid image having a predetermined density so as to output an image pattern having a uniform density all over the image concerned, it is desirable that the density of the toner image acquired by developing the latent image is constant and any density difference cannot be recognized in the reproduced image. However, in reality, the density of the toner image is fluctuated by variable factors, such as usage conditions of devices and consumable stores, environmental conditions, time variability, etc.
In order to suppress such the fluctuation, there has been widely employed in the image forming apparatus such a technology that a rectangular image pattern, which has a uniform density and is called a patch, is stored in advance, so as to change the image forming conditions based on the density of the toner image acquired by developing the latent image of the rectangular image pattern formed on the photoreceptor element.
In this connection, hereinafter, the image forming conditions mentioned in the above represents such conditions as a charging voltage, a developing bias, a toner density, etc., in regard to the toner image forming operation.
However, for instance, when a “Sweep shifting” phenomenon, in which a relatively large amount of toner are adhered onto an end edge portion of the toner image, occurs at the time of the developing operation, the density of the toner image acquired from the abovementioned patch cannot be uniform all over the toner image concerned. Accordingly, sometimes, it has become difficult to conduct an accurate density measurement.
The “sweep shifting” phenomenon, mentioned in the above, occurs remarkably in such an image forming apparatus that employs a developing bias voltage including a DC component and a AC component, which are superimposed with each other, so as to suppress the edge effect and to improve the mobility of the developing agent.
To cope with such the problem as mentioned in the above, for instance, Tokkaihei 7-175367 (Japanese Non-Examined Patent Publication) sets forth such a proposal that the toner image acquired from the patch is divided into plural areas, and the density measurement is performed for every divided area, so as to change the image forming conditions based on the detected density deviations of the toner image concerned.
With respect to a color image forming apparatus, it is necessary to accurately measure the density of the patched toner image corresponding to each of the primary colors, compared to the monochrome image forming apparatus, and sometimes, the abovementioned measurement becomes further severer.
For instance, in the color image forming apparatus employing the tandem method, the patched toner images respectively formed on the photoreceptor elements of the primary colors are sequentially transferred onto an intermediate transfer member, having a dense color, one by one.
The densities of the patched toner images aligned on the intermediate transfer member are detected at predetermined timings by a single patch sensor.
Accordingly, a wavelength sensitive range of the single patch sensor should cover such a range that is sufficient for detecting the densities of all colors represented by the patched toner images. Therefore, the S/N ratio (Signal to Noise ratio) of the detected signal acquired by the single patch sensor is liable to deteriorate, compared to that in such a case that an individual patch sensor is provided for each of the primary colors serving as the detecting objects.
Further, since colors of most of all intermediate transfer members are dense colors, for instance, a deep green color, a dense color near a black color or the like, a density difference, between density in an area to which the toner are not attached and a patch area to which the toner are attached, approaches to a smaller value. Accordingly, there has been a problem that the dynamic range for the detecting operation also becomes small.
Therefore, it is desirable that the density of the patched toner image, formed and developed on each of the photoreceptor element corresponding to each of the primary colors, is measured by the individual patch sensor before transferring it onto the intermediate transfer member, or the density of each of the patched toner images, transferred onto the intermediate transfer member, is measured by the individual patch sensor provided corresponding to each of the primary colors, so as to improve the accuracy of the measurement.
The abovementioned technology, however, would yield another problem that the cost of the apparatus increases, and/or its adjusting operation becomes complicated and cumbersome, and therefore, is not necessary employed as a good countermeasure.